The present invention relates to an improved edge dam or edge seal for use with applicators of the trailing blade type for applying a coating liquid to a moving web of paper.
Conventional applicators of the trailing blade type include means for applying coating material to a paper web that is usually supported and carried by a backing roll. Such applicators include a chamber having an opening extending across and parallel to the web, together with a doctor blade located at a trailing side of the opening for leveling the coating and a front wall or orifice plate extending from a leading side of the opening toward the web and defining an orifice for exit of excess coating material from an application zone between the wall and doctor blade. Coating liquid is supplied to the chamber and thence through the chamber opening to the application zone, and to seal the end spaces between the front wall and doctor blade to prevent escape of coating material laterally of the web, edge dams or edge seals are provided thereat.
For applicators of the foregoing type, the edge seal often comprises a solid bar of plastic, usually nylon, machined to fill a somewhat triangular shaped opening at either end of the coating zone defined by the doctor blade, orifice plate and backing roll. Ideally, the edge seal absolutely prevents leakage of any coating liquid to exterior of the applicator. However, that can require that the edge seal be in contact with the backing roll or web, and wear or abrasion of the seal and backing roll or marking and tearing of the web may occur. Consequently, the edge seal is only positioned closely adjacent to, but spaced from, the backing roll, and coating liquid seeps therepast.
In use of edge seals, an effort is made to minimize leakage of coating liquid. However, because conventional edge seals are relatively rigid, it is usually very difficult if not impossible to mount the same at the side ends of the application zone in sufficiently close proximity with the backing roll to allow only an acceptably small amount of leakage. Thus, in practice excessive coating liquid often leaks past the edge seals and the backing roll and/or paper web edges become contaminated with coating material.
If leakage of coating liquid past edge seals could be eliminated or at least significantly minimized, it would then be possible to coat only the center portion of the paper web, leaving narrow uncoated strips along the side edges of the web. This would advantageously reduce or eliminate coating material buildup on backing rolls following the coater as well as edge stickers on the coater backing roll. Unfortunately, conventional edges seals do not ordinarily permit the foregoing to be accomplished.